1. Field of the Invention
The present invention relates to a driving circuit for a display device which can be tested with high accuracy and a method of testing the display device.
2. Description of Related Art
In general, display drivers for driving a display device such as a liquid crystal display includes a shift register, a data register, a data latch, a level shifter, a digital-analog converter (D/A converter), and an output amplifier. The shift register sequentially shifts gray scale data of input digital image signals of each pixel. The data register sequentially holds the gray scale data corresponding to one scanning line. The data latch latches the gray scale data corresponding to one scanning line. The level shifter changes a voltage level of the gray scale data. The D/A converter performs D/A conversion on the gray scale data to generate an analog signal corresponding to the gray scale data. The analog signal is appropriately amplified by the output amplifier and then output.
Incidentally, the display driver has many D/A converters for driving each of pixels corresponding to one scanning line, so it is very complicated to test the driver for testing whether or not the converters normally operate. To that end, a method of testing a driver circuit that aims at testing the circuit for leak current from an analog voltage lead and output lead as short a period as possible over a wide range is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2002-304164 (hereinafter referred to as “Related Art”).
FIG. 9 shows the driver circuit described in the Related Art. As shown in FIG. 9, a driver circuit 101 includes a voltage generator 107, leads Mi connected to the voltage generator 107, a first switching device 102 connected with all the leads Mi, and N output stages AN connected with all the leads Mi. The N output stages AN each include a multiplexing device (hereinafter referred to as “selection circuit”) 104 connected with all the leads Mi, an amplifier 105 connected with the selection circuit 104, and a second switching device 103 connected between the selection circuit 104 and the amplifier 105 and connecting an output of the selection circuit 104 to the ground GND. The selection circuit 104 receives a digital signal EN to selectively output a signal of any one of the leads Mi.
The way to test the leakage between adjacent ones of the leads Mi in the driver circuit is as below. That is, M bus lines are charged up to the maximum potential, and the bus line Mi is disconnected from the voltage generator 107 by way of the first switching device 102 to bring all the leads Mi into a floating state. Then, one of the second switching devices 103 is selected to connect one of the output stages AN with one GND terminal. Then, a digital signal Ei is input to the output stage connected with the ground GND to switch any one of the leads Mi to GND. An output voltage of the other output stages is checked to thereby detect the leakage between the leads Mi and Mi−1, or between the leads Mi and Mi+1. It is thus possible to test the driver using the digital signal, so a test period can be reduced.
Incidentally, upon testing the driver for the function besides the test for the leakage between the leads, it is determine whether or not the driver is accepted through a speed test (through rate test) of a selection circuit (ROM unit) provided at a stage previous to the amplifier 105. In the speed test, the output voltage level is sampled over a predetermined period to check whether or not the through rate is a preset period or more. As a result, abnormalities in the ON resistance of the selection circuit and in the driving power of the output amplifier can be detected. In this case, in general, the selection circuit 104 selects a predetermined voltage from voltages generated by the voltage generator 107 to check the output voltage of the amplifier (AMP) 105.
However, in the test method described in the Related Art, the first switching device 102 is first connected with the voltage generator 107 and charged up to a predetermined potential, and then the second switching device 103 is turned off. Next, a target one of the leads Mi is selected based on the digital signal Ei and connected with GND. Finally, a potential of each of the output stages AN that output potentials of the other leads is checked. These series of steps should be repeated as many times as the number of leads Mi, so it takes much time to execute the test.
Further, in the above speed test of the selection circuit, a delay time (ROM speed) at the time of selecting the output voltage at the selection circuit 104 is generated by way of the amplifier 105, so the determination thereof is difficult. That is, in the speed test, characteristics of the ON resistance in the selection circuit 104 and characteristics of the amplifier 105 influence each other, and thus are hardly distinguished from each other. FIG. 10 is a schematic diagram showing an output voltage of the amplifier in the conventional driver circuit. As shown in FIG. 10, it is necessary to execute sampling during a transitional period for measuring the through rate. Hence, if the original output voltage VOUT1 is VOUT2, it is impossible to determine whether the problem is an insufficient ability of the AMP (amplifier) or a high ON resistance of the selection circuit 104.
That is, since the output voltage of the selection circuit 104 cannot be directly measured, it is difficult to set a determination criterion at sampling points. In addition, if a current leaks around the input side of the amplifier 105 of the selection circuit 104, and a voltage applied to the transistor drops, so the leakage of the current cannot be detected. Further, the problem is caused by the fact that the ON resistance in the selection circuit 104 is converted into the through rate of the output and a voltage level thereof is detected.
As mentioned above, although the driving circuit needs a variety of functional tests, in the test method described in, for example, the Related Art, only the leakage between leads can be tested. If a variety of tests such as the speed test of the selection circuit can be accurately and quickly executed in addition to a particular test of the leakage between leads in the driver circuit, a display device of, for example, high performance and low cost can be more easily provided.